How are you up to date with your OS and Motion? Try deleting your user preferences
~/Library/Preferences/com.apple.motion.plist
This is for running really slow.
It all depends on your Project. Our baseline (minimum) Macs are Dual 2.93GHz 6 Core from 32GB with ATi Radeon HD 5870 1GB cards.
Motion will still put these Macs on it's knees. It is the way it is designed.
This is how Motion works, it uses the RAM to cache all objects in your preview, the Processor is used to calculate the math for the curves/paths, behaviors and particles, the number crunching jobs and the VRAM represents the frames of your project.
As you can see the Graphic card can be the Achilles heel. You just have to work smart with your projects, for example hide the layers that you already have set and don't need to preview building your composite. Have your particles displayed as points... when your done designing them.
Don't forget you can work in proxies modes (Resolution) under the Render settings Full, Half, Third or Quarter plus other option like Motion Blur or the quality.
I hope this helps. Still have any question let them out.